How exactly does Asthma affect the body? To understand the mechanism of asthma, it is necessary to have an idea about the normal functioning of the respiratory system and how this disease disrupts this activity.
How Does The Normal Airway Function?
Airways connect the lungs with the external environment. Extra-pulmonary or upper airway connection includes trachea, larynx, nasopharynx and external nares through which the air comes in and goes out of the body. Intro-pulmonary or lower airways include the bronchial tree. This consists of two bronchi, the right and the left, one for each lung. The bronchi are further sub-divided into bronchioles. There are two types of bronchioles – membranous and respiratory. Gaseous exchange takes place at the respiratory bronchioles level.
A normal respiratory system lining consists of cilliary epithelial cells. The cillia propel any foreign body or particle upwards, towards the pharynx. This helps in the removal of any harmful foreign particles. There are some glands situated in the bronchial walls. These glands secrete water, electrolyte, mucin etc. in the lumen. There are various mediators including inflammatory mediators that control these secretions.
There are different cells present in the airway, each having some specific functions in the normal respiratory activity. These are goblet cells which secrete mucin, basophil cells, lymphocytes, mast cells and smooth muscle cells.
Structural Changes In Airways During Asthma
Asthma causes disruption in the epithelial cell lining. Epithelial cells desquamation or de-nucleation is the most common change. Sometimes the goblet cells secreting mucin increase in number. This is known as hyperplasia. This results in increased mucin production, causing a mucus plug. An increase in the number of eosinophilic cells in the bronchial wall as also in blood and sputum is a characteristic feature of asthma. Blood vessels to the bronchial tree also increase in number and size due to inflammation. These changes are more prominent and severe during the asthmatic attacks.
Development Of Clinical Manifestation Of Asthma
The basic problem in asthma is the inflammation of the bronchial tree and the breach in the respiratory epithelium. Because of inflammation, various changes take place in the airway giving rise to clinical manifestations. Broncho-constriction causing dyspnoea, edematous changes in the bronchial wall, increased mucus production resulting in cough are a few of these.
Allergic airway inflammation begins with inhalation and processing of allergens. Macrophages help in antigen presentation to airways. The smooth muscles in the airways undergo shortening and contraction, thus contributing to bronchial hyperactivity and resistance in the airways. Mucin secretion from the goblet cells of the airways increases and there is a leakage of plasma proteins from the bronchial blood vessels into the lumen. Mucocilliary clearance decreases. This adversely increases clinical features. The thickening of the airway wall occurs due to cellular and inflammatory changes. This results in narrowing of the airways. Mucus plug also adds to the obstruction in the respiratory passage. This, in turn, affects the gaseous exchange and results in mismatch of ventilation. This ultimately results in hypocapnia and increased respiratory drive thus producing an asthma attack.